A microphone is an electro-acoustical transducer. A transducer is a device that transfers energy from one system to another, usually in a different form, thus a microphone converts acoustical energy into electrical energy. Microphones originated with the development of telephony, but they were not used for sound recording until the late 1920's.
Modern microphones have evolved using just about every known means of acoustical-to-electrical transduction. Presently they have settled on magnetic and capacitor types due to their inherent flat frequency response. There are four basic types of microphones in practical use today: dynamic, condenser, carbon, and piezoelectric.
By far the most common type of microphone in use today is the dynamic, which operates on the principle of magnetic induction. Induction occurs when a conductor is moved within a magnetic field, inducing a voltage across the conductor according to the equation
e = the induced potential, in volts;
B = the magnetic flux density, in teslas;
l = the conductor length, in meters;
v = the velocity of movement, in meters-per-second.
This principle is often demonstrated by wrapping a wire around a metal rod and attaching each end of the wire to the terminals of a battery, thus magnetizing the rod and allowing it to pick up other metal objects such as paper clips, tacks, etc.
Two types of dynamic mics exist: the moving-coil and the ribbon. Moving-coil mics consist of a flexibly mounted diaphragm coupled to a fine coil of wire. The coil is mounted in the air gap of a magnet such that it is free to move back and forth within the gap. As sound strikes the diaphragm, it vibrates in response, causing the coil to cut through the magnetic field lines in the gap, inducing a small electrical current in the wire. The direction and magnitude of the current is directly related to the movement of the coil, thus the current is an electrical representation of the sound wave. Moving-coil mics are capable of relatively high, linear, low-distortion output levels. They are extremely dependable, rugged, and reliable.
Ribbon microphones also operate on the principle of magnetic induction. A very thin, corrugated, lightweight metal ribbon is suspended in the air gap of a powerful magnet. The ribbon is fixed at the ends but is free to vibrate in the middle. As sound strikes the ribbon, it vibrates in response, cutting the magnetic field lines of force in the air gap generating a small voltage in the ribbon. This voltage is so small that all ribbon microphones must employ a step-up transformer, which boosts the voltage as well as isolating the ribbon impedance from the load presented by the input to which the mic is connected. Ribbon microphones typically have excellent sonic characteristics with great warmth and gentle high frequency response.
The next most common microphone type is the condenser, which is comprised of two plates of a capacitor, one of which is stationary, the other of which is a thin, taut, metallized plastic diaphragm. A capacitor is a circuit element which stores a charge when a voltage is applied across it according to the equation
Q = CV
Q = the charge, in coulombs
C = capacitance, in farads
V = potential, in volts.
Capacitor mics need an external polarizing voltage (typically referred to as phantom power, usually +48 volts DC) to charge the diaphragm with a fixed, static voltage. As sound waves strike the diaphragm, it moves farther and closer to the back plate, changing the capacitance. This results in a corresponding variation in voltage according to the equation
V + ΔV = Q / (C + ΔC)
This fluctuating voltage is therefore an electrical representation of the sound wave. Since a coil of wire does not encumber the diaphragm of a condenser, it responds very quickly and accurately to incident sound. Condenser mics therefore have excellent sonic characteristics and are used extensively in recording. They are much more sensitive to environmental conditions such as humidity and physical shock than are dynamic microphones.
There exists a special class of condenser called the electret. Electret diaphragms are made of a unique plastic material that retains a static charge indefinitely without the need for an external polarizing voltage. These mics are characterized by excellent frequency response and long-term stability.
The carbon microphone operates on the principle of varying the resistance of loosely packed pulverized carbon granules as they react under the varying pressure of sound waves. These granules are packed into a small cup enclosed at one end by a brass disk attached to a metal diaphragm. As sound waves strike the diaphragm, it compresses and rarefies the granules causing a corresponding change in their resistance. A battery provides an activating voltage across the granules, and as their resistance changes so does this voltage, which is therefore an electrical representation of the sound wave. Carbon microphones are very limited in frequency response and dynamic range, which actually work to their advantage in telephony, the main use of carbon mics.
Some crystalline substances, when bent or twisted along certain of their axes generate a voltage between opposite faces. This effect is known as piezo-electricity (from the Greek piezin meaning, "to press"). A flexible diaphragm can be coupled to a crystal. As sound waves strike the diaphragm, it deforms the crystal causing a corresponding fluctuation in the voltage on opposing faces, which is therefore an electrical representation of the sound wave. Piezoelectric mics are not generally known for their sound quality, but are quite effective in contact pickups, which are crystals mounted directly to a source such as a piano soundboard.
"Sound Reinforcement Handbook" Gary Davis & Ralph Jones 1989
"The Microphone Handbook" John Eargle